Since the advent of a resist for KrF excimer laser (248 nm), an image forming method called chemical amplification is used as an image forming method for a resist so as to compensate for sensitivity reduction caused by light absorption. For example, the image forming method by positive chemical amplification is an image forming method of decomposing an acid generator in the exposed area upon exposure with excimer laser, electron beam, extreme-ultraviolet light or the like to produce an acid, converting an alkali-insoluble group into an alkali-soluble group by using the generated acid as a reaction catalyst in the baking after exposure (PEB: Post Exposure Bake), and removing the exposed area with an alkali developer.
As for the alkali developer used in the method above, various alkali developers have been proposed, but an aqueous alkali developer of 2.38 mass % TMAH (an aqueous tetramethylammonium hydroxide solution) is being used for general purposes.
On the other hand, as well as the currently predominant positive resist, formation of a fine pattern by a negative image is also being developed (see, for example, JP-A-2010-40849 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-2008-292975, JP-A-2010-217884). Because, in the production of a semiconductor device or the like, patterns having various profiles such as line, trench and hole need to be formed and some patterns are difficult to form by the current positive resist.
In the leading-edge pattern formation method of which development is being recently promoted, not only a positive pattern but also a negative pattern can be formed with high resolution by using an ArF excimer laser and further performing immersion exposure. A resin having an aromatic ring or a double bond has absorption for light at 193 nm that is ArF light, and cannot ensure sufficient transmittance. Therefore, the resin in the resist composition for ArF exposure is composed of an aliphatic component in many cases. However, it is difficult to design a resin composed of an aliphatic component to enhance the carbon density, that is, to reduce a so-call Ohnishi parameter, and this is disadvantageous in terms of etching resistance (see, for example, H. Gokan, S. Esho and Y. Ohnishi, J. Electrochem. Soc. 143, 130 (1983)).
Also, in the production of a semiconductor, not only ultimate refinement is required, but also, in view of effective utilization of existing equipment, it is being studied to substitute KrF exposure for a part of the process which has been conventionally performed by ArF exposure, but this opposes the historical background that the ArF exposure technique has been developed to surpass the limit of KrF exposure, and such substitution of KrF exposure for a part of the ArF exposure process may not merely face a refinement task but also involve various problems associated with, for example, improvement of materials (such as resin) used and difference in the exposure mechanism, which are technically difficult to solve.
In addition, as an application of the above-described resist technology, there is evolving usage for microfabrication, such as application to ion implantation where a resist composition is used when implanting ion (charge injection), which is one step of the logic device fabrication or the like.
In the case of using a resist composition for ion implantation, the resist composition is sometimes coated, exposed and developed on a substrate having previously formed thereon a pattern (hereinafter, referred to as a stepped substrate), and microfabrication on a stepped substrate is required.
However, the profile of the obtained pattern may be impaired by the effect of a standing wave generated due to reflection of exposure light from the substrate or by the diffused reflection of exposure light due to a stepped portion in the stepped substrate.
Also, a method of providing an anti-reflective film (Bottom Anti-Reflective Coating; BARC) between the resist film and the substrate is known, but when an anti-reflective film is provided, particularly in the case of using the resist composition for the application to ion implantation, a step of removing the anti-reflective film by etching is required before implanting an ion, and this incurs an increase in the production cost.